Automobile charging systems generally include an alternator and a battery. The alternator includes a rotor coil, stator, rectifier bridge, diode trio and voltage regulator.
A magnetic field is produced by passing a current through the rotor coil of the alternator. A voltage is induced in the stator of an alternator by current in the rotor coil when the alternator shaft is rotated by the automobile's engine. The rectifier bridge converts the AC voltage induced in the stator to a DC voltage needed for charging the automobile's battery. A diode trio further converts the AC voltage induced in the stator to a DC voltage for supplying current to the rotor coil. The voltage regulator controls the current in the rotor coil in order to maintain the output voltage of the alternator at a desired constant level.
Automotive alternators can generally divided into two types. In a first type, the alternator rotor coil is attached between the system power supply, or battery, and a power MOSFET connecting the rotor coil to ground. In a second type, the alternator rotor coil is connected between ground and a power MOSFET connecting the rotor coil to the system power supply, or battery.
FIG. 1 shows components of the first type of automotive alternator. In FIG. 1, the source of a power MOSFET 101 is connected directly to ground and the drain is connected to the rotor coil 103 of the automotive alternator. It is reasonably easy to drive this type of automotive alternator. The voltage applied to the gate of MOSFET 101 need only be reasonably higher than its source by a threshold voltage (Vt) for the power MOSFET 101 to turn on in a “low resistance” region of operation. The threshold voltage is the minimum voltage which must be applied between the gate and source of the transistor in order to enable current flow from the drain to the source. A threshold voltage of Vt (˜1V) will put the MOSFET 101 in a “low resistance” mode since its source is grounded, as shown in FIG. 1. The system voltage from the application specific integrated circuit (ASIC) 102, VccASIC, is typically around 5V. Thus, standard circuitry as provided in the ASIC 102 can be used to apply the MOSFET 101 gate voltage to cause a “low resistance” operation mode to enable current to flow through the rotor coil 103 of the automotive alternator.
FIG. 2 shows components of the second type of automotive alternator. In FIG. 2, the source of a power MOSFET 201 is connected to the rotor coil 203. The drain of the power MOSFET is connected to the alternator system voltage Vcc. This type of automotive alternator is more difficult to drive than the first type of automotive alternator shown in FIG. 1.
As with FIG. 1, the power MOSFET transitor 203 of FIG. 2 also has a minimum threshold voltage which must be applied to its gate in order to place the transistor into the “low resistance” region of operation. In order to turn on a power MOSFET 203 with a load connected on the source side, a gate voltage higher than the drain potential must be applied.
Since a potential exists across the rotor coil 203 of the alternator system voltage Vcc in FIG. 2, the voltage applied to the gate of the power MOSFET 201 must be higher than the system voltage in order to put the power MOSFET into the “low resistance” region of operation. The alternator system voltage Vcc ranges up to 14.4V. The alternator voltage Vcc is typically the battery voltage of 12V, or 14.4 when the alternator is running. To put the MOSFET 201 into a “low resistance” mode, the gate voltage applied to MOSFET 101 must be higher than both the source and drain voltages by a threshold voltage Vt (˜1V). With a Vt needed to put the MOSFET 201 in a “low resistance” region the gate voltage applied to the MOSFET 201 will then typically be at least 15.4V.
Typically, the electronics used to drive a MOSFET gate are embedded in an ASIC 202. Since a higher voltage than the alternator system voltage of 15.4V is needed to drive the gate of the power MOSFET 203, the ASIC will require a charge pump circuit for generating the necessary voltage. Since the ASIC system voltage VccASIC is around 5V, the ASIC outputs cannot be connected to the gate of the MOSFET 201 directly unless a charge pump is included to supply the increased voltage. Charge pump circuits within the ASIC will significantly increase the cost of the ASIC 202.